In the semiconductor industry especially, and in other industries as well, extremely severe conditions are encountered in process equipment because of highly corrosive atmospheric conditions, or environment, which exist at or adjacent to processing equipment. Such corrosive atmospheric environments are extremely hard on monitoring equipment, particularly because metals which are conventionally used in such monitoring devices cannot reliably stand up to the corrosive environment for long periods of time so that the equipment may be safely and efficiently operated throughout the service life of the equipment.
The highly corrosive environment is created by one or more hazardous chemicals which are approved for use in the semiconductor industry, and which may exist in liquid or gaseous form, alone or in combination with other of such chemicals. A partial list of such chemicals includes those identified as follows:
______________________________________ Acetic Acid Acetone Ammonium Fluoride Ammonium Hydroxide Antimony Potassium Tartrate ceric ammonium nitrate ceric ammonium nitrate Choline EKC 130-photoresist stripper Ethylene Glycol Ferric chloride Hydrocholoric Acid Hydrofluoric Acid Hydrogen Peroxide IPA-isopropyl alcohol Methanol n-Butyl Acetate Nitric Acid PBR 1 Phosphoric Acid Potassium ferrocyanide Potassium Hydroxide potassium phosphate PRS 1000 Sodium Hydroxide Sulfuric Acid Tannic Acid TMAH-tetramethylammonium hydroxide ______________________________________
These various chemicals must be delivered to processing equipment, and in order to do so, liquid transporting systems carry the liquid chemicals from replaceable supply tanks through pumping and regulating stations and through temporary storage facilities so that the chemicals are readily available at the processing stations. Of course, such liquid chemical transport systems must involve pipes and tubes, valves and fittings, oftentimes filters and flow meters, and related devices, most of which are made of plastics resistant to the deteriorating effects of harsh chemicals. Of course, anything mechanical is subject to potential leakage, and such leakage can create extremely hazardous conditions, both to the processing of semiconductor wafers or other products, and also to personnel who may have to tend and maintain the processing equipment and transport systems.
Although capacitance proximity devices such as switches have been successfully used in particular safe environments for determining existence of liquids or objects, and have used sheet metal sensors remote from the operating circuitry and proximity switch, such capacitance proximity devices have not been successfully used in corrosive environments because of the lack of reliability.